Figure 3: a) FT-IR spectra of the four NSs. b) Water
contact angle of natural nut shells (NNS) and treated nut shells (NSs).
c) Photograph of i) natural WS, ii) treated WS, and iii) nano-engineered
WS-H+. Confocal Laser Scanning Microscopy (CLSM) of
the d) untreated natural walnut shell (NWS), e) treated WS and f)
WS-H+ indicating the lignin reductions (color changes
from green to blue). SEM image of the g) porous, thin layer of inner
shell surface of WS-H+, h) nanostructured outer shell
surface of WS-H+ indicating the random pore diameter
of 78 nm and 190 nm, i) WS-H+ after dipping in
alkaline reservoir (pH 12.5) for 8 h with a random pore diameter of 160
nm.
The CLSM images of Figure 3 (d, e, f) confirms the variations
in lignin contents in WS. The lignin (green) tends to disappear after
undergoing the chemical treatments, and more cellulose (blue) gets
exposed, which indicates the partial delignification of the WS. SEM
images of WS-H+, shown in Figure 3 (h) confirms the enhanced
number of nanopores after the nanoengineering of the WS. A prominently
porous layer on the interior surface was also observed in Figure
3 (g). Figure 3 (i) confirms the structural changes after
dipping into the alkaline reservoir for 8 h, whereas the nanopores
sustained.
Electricity generation and
performance
analyses
Herein, water droplet evaporation-based hydrovoltaic electricity
generation has been investigated utilizing the nutshells. A sandwich
structure of graphite-nutshell-graphite (G-NS-G) was utilized for the
Open Circuit Voltage (Voc), Short Circuit Current
(Isc), and Short Circuit Current density
(Jsc) analyses. The typical porous NS structure, as
schematically illustrated in Figure 4 (a), facilitates the
water flow through the micro/nanochannels with multiple functional
groups. When the DI water contacts this porous structure with abundant
hydrophilic functional groups, water droplets are adsorbed and
transferred through the micro/nanochannels under capillary action. At
the same time, this abundant oxygenated functional group of the NS
surface interacts with water and generates negative surface charges.
water is freely transferred throughout these channels under capillary
actions and the continual evaporation helps to circulate the water in
the direction of water evaporation. The transportation duration of water
from the bottom surface to the top surface is different among these four
NS, which is depicted in Figure 4 (b). A very porous
feature of AS along its surface and cross sections, confirmed byFigure-S 3 (a) causes the faster movement of water compared to
other NSs.